A soldering method is widely used as a technique for bonding the electrodes of different electronic components with each other, such as surface mounting of a chip-type electronic component onto a substrate.
Further, as a bonding material, a solder paste containing a solder, a thermosetting resin, an activating agent, and others is known from the past.
For example, Patent Document 1 proposes a solder paste containing solder particles made of a tin-bismuth-based solder, a thermosetting resin that is cured at a temperature higher than the melting point of the solder, and an activating agent that is activated by the heat at the time of soldering to remove an oxide film on the surface of the solder, wherein the activating agent is activated at a temperature equal to or higher than the melting point of the solder.
FIG. 4 is a cross-sectional view for describing the soldering method disclosed in Patent Document 1, where a chip-type electronic component 101 is surface-mounted on a printed substrate 102. In other words, in this chip-type electronic component 101, external electrodes 104a, 104b are formed on the two sides of a component element body 103. On the other hand, in the printed substrate 102, a pair of land electrodes 105a, 105b is formed on the surface to correspond to the external electrodes 104a, 104b, respectively. Further, a solder paste containing a solder, a thermosetting resin, and an activating agent is used as a bonding material, whereby the land electrodes 105a, 105b and the external electrodes 104a, 104b are bonded via solders 106a, 106b, respectively.
In other words, if the thermosetting resin starts to be cured before the solder particles are melted, the melted solder cannot flow to move sufficiently in the thermosetting resin. As a result of this, the solder particles may not be bonded with each other or the solder may not be bonded to the electrodes, thereby raising a fear of deteriorating the electrical connection property of the electronic components with each other. Furthermore, the activating agent must be melted to be brought into contact with the surface of the solder particles before the thermosetting resin is cured to lose its fluidity.
Therefore, in Patent Document 1, by using a solder paste in which the curing temperature of the thermosetting resin is higher than the melting point of the solder particles or the activation temperature of the activating agent, the solder 106a, 106b being melted is allowed to flow and move sufficiently in a thermosetting resin 107a, 107b, and the activating agent is brought into contact with the solder particles, whereby the activating agent and the solder particles undergo an oxidation-reduction reaction on the surface of the solder particles thereby to remove an oxide layer that is present on the surface of the solder particles.
Also, in Patent Document 1, the activation temperature of the activating agent is made to be equal to or higher than the melting point of the solder particles, thereby to delay the curing action of the thermosetting resin 107a, 107b. In other words, by raising the activation temperature of the activating agent which acts also as a curing agent of the thermosetting resin 107a, 107b, the curing action of the thermosetting resin 107a, 107b is delayed, thereby to prevent the fluidity of the solder 106a, 106b being melted from being hindered by the thermosetting resin 107a, 107b, so as to prevent decrease in the self-alignment.    Patent Document 1: Japanese Patent Application Laid-open (JP-A) No. 2006-150413 (Claim 1, paragraphs [0013], [0031], FIG. 1, and others)